You passed an argument sequence to one of the sum or avg functions that contains values that are not numbers or durations, or that are a mixture of numbers and durations.

You passed an argument sequence to one of the min or max functions that contains values that do not support the < and > operators, or that have a mixture of different types.

You attempted to find the effective Boolean value of a multi-item sequence whose first item is an atomic value. Effective Boolean value is calculated when calling the boolean and not functions, but also in many different operations that do not involve functions, such as conditional (if-then-else) expressions, where clauses of FLWORs, and predicates of path expressions. Effective Boolean value is discussed in more detail in "Effective Boolean Value" in Chapter 11.